Flexible metal-laminated boards are used primarily as substrates for flexible printed wiring boards, and the recent trend toward smaller sizes, better portability, and higher density in electronic devices with printed wiring boards is growing, and this has been accompanied by increasing demand for finer pitch and higher dielectric properties in printed wiring boards. As a result, there has been more and more development of adhesive-free flexible metal-laminated boards having two-layered structures composed of a base and metal foil.
For example, techniques are available for producing laminated metal (e.g., copper) boards having a two-layered structure by a variety of methods, such as a method in which a thin metal layer is formed directly on a polyimide film by a metallizing process such as sputtering deposition or ion plating, and the metal foil is then thickened to the necessary thickness by electrolytic plating, or a method in which a polyimide resin is made into a varnish and allowed to adhere to metal foil by casting, lamination, or the like.
Even though metallizing methods allow ultra-thin metal foil to be easily formed and are suitable for producing a finer pitch, a problem is that production costs are greater because of the need for large vacuum sputtering equipment or the like.
Also, casting and laminating methods make use of an anchoring effect in the adhesion between metal foil and polyimide, and the surface of copper foil must therefore be rough. On the other hand, a problem is that such surface roughness leads to the deterioration of high dielectric properties and makes it impossible to sufficiently obtain the high frequency properties that are necessary in some printed wiring boards and the like. In addition, because existing metal foil is used, the metal foil thickness is limited, and as such is unsuitable for achieving a fine pitch.
To overcome such problems, a technique has been proposed, wherein a polymer layer, to which a polymer containing an electroless plated catalyst metal or precursor thereof has been directly chemically bonded, is laminated onto a polyimide film, and an electroless plating process is then carried out, so as to convert the polymer layer to an organic/inorganic hybrid layer and obtain a thin copper-clad polyimide film endowed with adhesive properties (such as Patent Citation 1).
Another method has also been proposed, wherein surface treatment of a polyimide film to form a surface roughness of 5 to 50 nm is followed by alkali treatment, the provision of metal ions, a reducing treatment, and plating (such as Patent Citation 2).
Patent Citation 1: Unexamined Patent Application 2006-193780
Patent Citation 2: Unexamined Patent Application 2002-256443